1. Field of the Invention
The present invention relates to a light emitting device including a semiconductor light emitting element that emits primary light and a wavelength conversion unit including a phosphor that absorbs the primary light and emits secondary light.
2. Description of the Related Art
In recent years, illuminators that use light emitting diode (LED) elements have been receiving attention. Such light emitting diode elements have excellent characteristics such as power saving, a long product lifetime, and low impact on the environment. In particular, various light emitting devices including a light emitting device that emits white light have been developed by combining a phosphor with a light emitting diode element that emits blue light or ultraviolet light. A phosphor that emits light having a desired wavelength is excited by light emitted from the light emitting diode element and emits such light. It is hoped that such a light emitting device will replace incandescent lamps or fluorescent lamps as an illuminator. Thus, such a light emitting device has been actively developed.
Japanese Unexamined Patent Application Publication No. 2007-49114 (Patent Document 1) discloses an example of a light emitting device that uses such a light emitting diode. FIG. 8 is a schematic view of the light emitting device disclosed in Patent Document 1. In Patent Document 1, a light emitting device 80 includes a light emitting element 81 that emits primary light and a wavelength conversion unit 82 that absorbs part of the primary light and emits secondary light having a wavelength longer than or equal to that of the primary light. The wavelength conversion unit 82 includes a plurality of types of phosphors 83, 84, and 85 having light absorption bands different from each other. At least one of the plurality of types of phosphors has an absorption band in which secondary light emitted from at least one of the other phosphors can be absorbed. In such a structure, emission color is easily set and a light emitting device with high luminance can be realized.
In light emitting devices that use light emitting diode elements, a rare-earth activated phosphor has been mainly used so far. In recent years, a semiconductor nanocrystal phosphor (hereinafter referred to as “nanocrystal phosphor”) has been receiving attention as a fluorescent material that allows production of a light emitting device having good color rendering properties and high emission efficiency. A direct band gap semiconductor emits fluorescence having a wavelength intrinsic to the semiconductor. As a result of decreasing the grain size of the semiconductor to a size approximately equivalent to a Bohr radius, the kinetic energy of electrons is increased and the energy level of the electrons becomes discrete in both a valence band and a conduction band. Consequently, the emission wavelength decreases as the grain size decreases. Therefore, a semiconductor nanocrystal phosphor is different from conventional phosphors in that the emission wavelength of the semiconductor nanocrystal phosphor can be freely controlled by changing its grain size. Furthermore, by combining some types of nanocrystal phosphors with each other, light having various wavelengths can be emitted. The following semiconductors have been mainly studied and reported as materials of a nanocrystal phosphor. An example of a group II-VI compound semiconductor is CdSe (e.g., refer to C. B. Murray, D. J. Norris, and M. G. Bawendi, Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites, “Journal of the American Chemical Society”, vol. 115, 1993, pp. 8706 to 8715). An example of a group III-V compound semiconductor is InP.
FIG. 9 is a schematic view of an illuminator disclosed in Japanese Unexamined Patent Application Publication No. 2004-71357 (Patent Document 2). In an illuminator 90 disclosed in Patent Document 2, three phosphors containing nanocrystals having different grain sizes are stacked on top of each other in descending order of grain size in a direction of an optical path so as to constitute a wavelength conversion unit 92. Specifically, a red phosphor 93 containing an InN-based nanocrystal that has the largest grain size and emits red light, a green phosphor 94 containing an InN-based nanocrystal that has a grain size smaller than that of the red phosphor 93 and emits green light, and a blue phosphor 95 containing an InN-based nanocrystal that has a grain size smaller than that of the green phosphor 94 and emits blue light are stacked on top of each other in that order from a light source 91 so as to constitute the wavelength conversion unit 92.
However, in the case where the light emitting device 80 disclosed in Patent Document 1 is actually operated to emit light, when light emitted from the light emitting element 81 passes through each of multiple layers composed of the phosphors, part of the light is absorbed by a layer located above. Thus, the efficiency of extracting red light from the lowermost layer composed of the phosphor 83 is particularly decreased. Furthermore, the emission efficiency of a phosphor is not necessarily 100%. Therefore, when two-step optical conversion (e.g., the phosphor 83 absorbs light emitted from the light emitting element 81 and generates fluorescence, and then the phosphor 84 absorbs the fluorescence and emits secondary light in the form of fluorescence) is performed, energy loss occurs. As a result, the intensity of light emitted from the lowermost layer composed of the phosphor 83 is particularly decreased. This disturbs the balance of emission intensities of the light colors and it becomes difficult to achieve ideal color reproducibility and luminosity. Note that the term “emission efficiency” mentioned herein is defined as a ratio of the number of photons emitted as photoluminescence relative to the number of photons absorbed.
A nanocrystal phosphor absorbs light having any wavelength shorter than its emission wavelength. Therefore, when nanocrystal phosphors having different grain sizes are stacked on top of each other in the illuminator 90 disclosed in Patent Document 2, a nanocrystal phosphor that emits secondary light having a longer wavelength needs to be disposed in a layer closer to the light emitting element 91 in the optical path to prevent secondary light having a shorter wavelength from being absorbed by the nanocrystal phosphor that emits secondary light having a longer wavelength.
In either case, a step of sealing phosphor-containing resins in a chip in a desired order is added to the production process, which decreases the production efficiency.